The new germ is always developed, in the first instance, at the side of the base of the old tooth, never in the cavity of the base ; the crocodiles form no exception to this rule. The poison-fangs of serpents succeed each other from behind forwards; in almost every other instance the germ of the successional tooth is developed at the inner side of the base of its predecessor. In the frog the dental germ makes its appearance in the form of a papilla developed from the bottom and towards the outer side of a small fissure in the mucous membrane or gum that fills up the shallow groove at the inner side of the alveolar parapet and its adherent teeth : the papilla is soon enveloped by a capsular pro cess of the surrounding membrane : there is a small enamel pulp developed from the cap sule opposite the apex of the tooth ; the de position of the earthy salts in this mould is accompanied by ossification of the capsule, which afterwards proceeds pari passu with the calcification of the dentinal papilla or pulp; so that, with the exception of its base, the surface of the uncalcified part of the pulp alone remains normally unadherent to the cap sule.
As the tooth acquires hardness and size, it presses against the base of the contiguous attached tooth, causes a progressive absorp tion of that part, and finally undermines, dis places, and replaces its predecessor. The number of nascent matrices of the successional teeth is so great in the frog, and they are crowded so close together, that it is not unusual to find the capsules of contiguous tooth-germs becoming adherent together, as their ossification proceeds. After a brief ma ceration, the soft gum may be stripped from the shallow alveolar depression, and the younger tooth-germs in different stages of growth are brought away with it.
The mode of development of the teeth of serpents does not differ essentially from that of the teeth of the Batrachian above described except in the relation of the papillm of the successional poison-fangs to the branch of the poison-duct that traverses the cavity of the loose mucous gum in which they are deve loped.
Batrachian modifications.— Some of the pe culiarities of the dentition of the Batrachians have already been noticed, as in the compa rison of the Siren with the Lepidosiren, in which the true amphibian was shown to have numerous teeth on the palate and lower jaw,* The piscine character of rasp-like teeth aggregated in numerous series, is manifested also in the Axolotl +, upon the palatal region of the mouth, and upon the splenial or oper cular element of the lower jaw ; but the superior maxillary bones are here developed, and also support teeth. The premandibular and the premaxillary bones, instead of pre serving the larval condition of the horny sheath, have their alveolar border armed with a single row of small, equal, fine and sharp pointed denticles, which are continued above, along the maxillaries ; thus establishing the commencement of the ordinary Batrachian condition of the marginal teeth of the buccal cavity. The dentigerous bones of the palate consist of two plates on each side, as in the Siren ; the anterior pair, or vomerine bones, converge and meet at their anterior extre mities; the minute denticles which they sup port are arranged quincuncially ; the posterior pair of bones are continued backwards ac cording to the usual disposition of the ptery goids, to abut against the tympanic bones ; the denticles are confined to the anterior part of their oral surface, and resemble in their arrangement and anchylosed attachment those of the vomerine series, of which they form the posterior termination.
The frogs (Rana) have no teeth on the lower jaw ; but in some species the alveolar edge of this bone is finely notched or dentated, as in the horned frogs (Ceratophrys). The in termaxillary and maxillary bones support a long, close-set, single series of small, conical, hollow teeth, of which the apices only project beyond the external alveolar ridge to which they are attached. A short transverse row of similar but smaller teeth extends along the posterior border of each vomer, except in the slender-armed frogs (Leptobrachium), and in some of the tree frogs (e. g. Euchnemis), in which the roof of the mouth is edentulous.
Amongst the most extraordinary examples of extinct reptiles are those which are charac. tensed by the labyrinthic modification of the dental structure above described, and which with some affinities to Saurians, combine characters which are essentially those of the order Batrachia. I have ascertained by fossil portions of the upper jaw of the Labyrinthodon leptognathus that the maxillary or facial divi sion of the skull was broad, much depressed, and flattened, resembling the skull of the gigantic Salamander and of the Alligator ; and the outer surface of the bones was strongly sculptured, as in the Crocodilian family, but of a relatively larger -and coarser pattern. 'The upper jaw contains a single row of small teeth, about sixty in number, anterior to which are three or four large conical tusks. The bases of the serial teeth project directly from the outer wall of the shallow socket, there being no alveolar ridge external to it. The second large anterior tusk is three times the size of the first of the serial teeth, and the size of these teeth gradually diminishes as they are placed further back ; the length of the common-sized teeth being about two lines, and the greatest breadth one-third of a line. The apical two-thirds of each tooth is smooth, but the basal third is fluted and anchylosed to the outer wall of the socket. The osseous roof of the mouth is principally composed of a pair of broad and flat bones, homologous with the divided vomer in Batrachia, but of much greater relative extent, approaching, in this respect, those of the Menopome, and de fending the mouth with a more extensive roof of bone than exists in any Lacertian reptile ; physiologically, therefore, the La byrinthodon, in this part of its structure, comes nearest to the Crocodile ; but the structure itself, morphologically, is essen tially Batrachian.* In the Menopornet and gigantic Salamander, a row of small teeth ex tends transversely across the anterior extre mity of the vomerine bones ; and the occur rence in the Labyrinthodon of a similar row, consisting in each palatine bone of three median small teeth and two outer larger ones, marks most strongly its Batrachian nature ; and from the outermost tooth a longitudinal row of small and equal-sized teeth is con tinued backward along the exterior margin of the palatine bone. The whole of this series of palatal teeth is nearly concentric with the maxillary teeth.